Light Below Deck

A Deck Prism from the Charles W. Morgan

Original content by: Dr. Paul Goodwin

zoomable artifact image here

About This Artifact

The below deck area of a ship is a very dark place.  Throughout history, many devices have been used to provide light to this space.  One of these is the deck prism (also known as a deck light).  This particular deck prism is said to be from the whaleship Charles W. Morgan.  A hexagonal cone, it is made from bluish-green-clear glass, and dates from the 1840s, the earliest known date for this design.  It is interesting to note that most of the companies that fabricated deck lights were actually iron works.

Deck prisms provided natural light from above, dispensing it below decks on a ship.  They were especially useful on wooden ships that carried dangerous cargo, such as gunpowder, coal, or whale oil, where oil or kerosene lamps or candles were always a fire hazard.  Also, in the event of a fire below decks, the deck prism would show the glow of the flames below, giving the crew more time to act.

The earliest deck lights were known as bull's eye lenses, which were flat on one side and convex on the other.  All early deck lights were subject to frequent damage.  Notice that this artifact is chipped, and has pieces missing.  In addition to breakage, which could result in a leak, early deck lights were often cemented in place.  Over time, the cement would crack and loosen, which allowed water to leak through the deck.

In 1861, Henry Lanergan patented a threaded deck light that could be screwed into a brass frame.  In the event of damage or wear, it could quickly be replaced — a marked improvement over earlier designs.  The hexagon pyramid style is still in use.

Currently, we can see a spin-off from deck lights in the use of translucent glass for sidewalk vault lighting.

Questions for Further Thought

  1. Why do you suppose the color of the deck light was a blue-green?
  2. How do you think a deck prism is made?
  3. Why is the deck light hexagonal in shape? Does shape play a role in how the light is displayed? Or is it purely decorative?